The invention relates to a process for the fine machining of the toothed gears of a gearbox.
The desire for weight reduction and noise reduction in motorized vehicles necessitates that also the gearboxes are manufactured more carefully and precisely. This leads to the necessity of having the gear teeth of gearbox gears, which until now had been often only shaved in the soft state and then hardened, undergo fine machining after hardening.
In conventional designs of today""s manual shift transmissions, more than 70% of all toothed gears are disc-shaped, that is, the diameter is larger than the width, and they usually have a cylindrical boring.
The commonly and widely used technology for the manufacture of such typical transmission gears of motorized vehicles is thus the following process:
machining of the unmachined part on a lathe, boring with grinding allowance,
rough forming of the gearing in the soft state with mounting in the pre-worked boring,
hardening
clamping of the gear in the teeth and grinding of the boring
clamping of the gear in the boring and gridding of the teeth
Both steps of the hard-fine machining are relatively elaborate and therefore expensive. That is also a reason why today still much gearing is shaved in the soft state to the finished dimensions instead of being hard fine-machined. The distortions produced during hardening are reluctantly accepted, while one attempts to minimize them using all possible countermeasures. As mentioned, however, in many modem transmissions, such gears which have only been shaved do not meet the requirements anymore.
The boring cannot remain without hard fine-machining, since mostly such a gear is guided with needle bearings on the gear shaft which requires a very precise and fine boring surface. For the machining of the boring, the workpiece is normally clamped in the pre-worked gearing to guarantee a good circular boring to the boring. Precisely this clamping in the gearing, however, is delicate and expensive. It is not only that the means for the clamping of each gear have to be manufactured separately, but also that due to their complexity and tendency to become dirty and deteriorate they are not liked in mass production.
A good circular running is in all cases necessary, regardless of whether or not the teeth are hard-fine machined afterwards. If, however, a hard fine-machining of the teeth is provided, then again an expensive clamping means for a highly precise fixing in the boring is necessary. Such clamping means are mostly designed as hydraulic expandable clamping arbor with a cylindrical mounting zone.
It is the object of the present invention to disclose a process with which the hard fine-machining of the transmission gears of motorized vehicles can be rationalized and improved and which manages with far simpler and less expensive clamping means.
According to the invention, the machining of the teeth and of the boring of gears is performed in the same fixing and simultaneously. In doing so, the fixing of the workpiece is designed in such a way that the surfaces to be machined remain freely accessible to the appropriate tools. Besides the time savings in the manufacturing, in doing so, automatically a minimum runout between the boring and the gearing is achieved; also the expensive fixing means for clamping the workpiece in the gearing as well as thereafter in the boring for the grinding or honing of the gearing are eliminated.
The implementation of the process according to the invention is made possible thanks to several specific characteristics on disc-shaped gears, as can be found in, for example, the transmissions of motorized vehicles, and the availability of the continuous machining process, for example for the continuous generating grinding process, which requires a rapid and continuous rotation of the workpiece for the machining process. The rotational movement is used simultaneous with the machining of the gear in order to machine the boring in a known manner, for example, through internal grinding or honing.
As mentioned above, according to current practice, the hardened workpieces are in most cases clamped in the gearing for the grinding of the boring. This is mainly for manufacturing-related technical reasons. On the one hand, such workpieces can hardly be fixed differently for this machining; on the other hand, the hard machining of the gearing to follow it is an elaborate and expensive process, and therefore one attempts to keep the machining allowance as small as possible. In order to obtain a proper grinding of all of the tooth flanks despite small machining allowance, it is thus necessary to machine the boring as precisely as necessary running concentric to the pre-machined gearing. It is obvious that the precision of such a clamping in the gearing has its limits, since the distortions by hardening cannot be avoided. Clearly functional reasons speak for the highly precise fixing of the workpieces in the boring for the fine machining of the gearing; it is exactly here where the running quality of the gear is to be produced.
If the workpiece is fixed in such a way that the gearing and the boring can be machined simultaneously, one can avoid the highly precise mounting because a perfect circular running between both of the functional surfaces is automatically produced.
Such a mounting can be achieved if both chamfers of the boring or one chamfer and a plane surface or the outer surface of a hub or a collar of the workpiece is used for centering by means of fitting centering pieces, wherein at least the centering piece on the side of the boring machining tool must have a through-boring.